In semiconductor device processing, oxides of silicon are used in many different forms for many applications. Dense, thermally grown oxides of silicon are typically used as the primary gate dielectric film in MOS (metal oxide-silicon) transistors. Steam grown thermal oxides are commonly used as a field oxidation dielectric layer. Undoped chemically deposited oxides, such as tetraethylorthosilicate derived oxide (TEOS), produced by wet or vapor (CVD) processes are other types of dense oxides commonly encountered.
Other forms of silicon oxide commonly encountered are porous. Examples include doped oxides such as phosphosilicate glass (PSG) and borophosphosilicate glass (BPSG), which are commonly used as inter-metal layer dielectrics because they can be easily planarized with an elevated temperature reflow process. Spin-on-glass (SOG) is another porous oxide used in dielectric applications where planarization is critical. An SOG is a siloxane-type polymer in an organic solvent which is deposited in liquid form and then cured at elevated temperature to form a solid silicon oxide film. Other porous silicon oxides commonly encountered include borosilicate glass (BSG), boron doped TEOS, phosphorous doped TEOS and boron/phosphorous doped TEOS (BP TEOS).
Many semiconductor device manufacturing processes require selective etching processes to allow for removal of one form of silicon oxide in preference to another form of silicon oxide or to another material.
In the case of selective etching of oxides, liquid hydrogen fluoride (HF) is typically not successful due to very similar wetting and etching characteristics of most oxides. It may be possible to enhance selectivity in some wet HF systems using buffered HF.
It is known, from U.S. Pat. No. 4,749,440, to use moist HF vapor to effect silicon oxide removal. Using a commercial embodiment of this technology sold under the Excalibur.RTM. brand by FSI International, Chaska, Minn., U.S.A., selectivities between specific oxides such as boron phosphorous silica glass (BPSG) and thermal oxide have been demonstrated up to 5:1.
Recently, it has been proposed to build a cylindrical capacitor device from a silicon substrate having a hollow cylinder of polysilicon with a depth of approximately 5,000 .ANG. thereon. This cylinder is built up on a dense silicon oxide layer, typically TEOS. In the manufacturing process, a cylinder filled with a porous oxide, typically BP TEOS, is produced on the undopeal TEOS oxide layer, leaving the TEOS oxide layer exposed around the outside of the cylinder base. The BP TEOS contents of the cylinder must then be removed without destroying the polysilicon walls and without over etching the exposed TEOS layer under the cylinder. To accomplish this selective removal task using a masking, etching and cleaning technique could require as many as 15 individual process steps.
There therefore exists a need for a gentle method of removing of a porous silicon oxide such as BP TEOS with extremely high selectivity relative to undoped silicon oxides such as TEOS. In particular, selectivities on the order of 50:1 to 100:1 are required.